1. Field of the Invention
The present invention relates to a process of preparing a positive active material for a lithium secondary battery, and more specifically, to a process of preparing a positive active material for a lithium secondary battery with high discharge potential, high power density, high rate capability, and good cycle life characteristics.
2. Description of the Related Art
Generally, rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated for positive and negative active materials. For the electrolyte, an organic solution of a lithium salt or a lithium ion-conducting polymer is used. A rechargeable lithium battery produces electrical energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of the lithium ions.
Among the active materials which have been considered for the active material of negative electrodes of batteries, lithium metal gives both high cell capacity and high voltage because it has a high electrical capacity per unit mass and relatively high electronegativity. However, since it is difficult to assure the safety of a battery using lithium metal, a carbonaceous material that is capable of intercalating and deintercalating lithium ions is used extensively for the active material of the negative electrodes in lithium secondary batteries.
Lithium metal compounds of a complex formula are often used as a positive active material of the lithium secondary battery. Typical examples include lithium-containing compounds such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), and LiMnO2. Manganese-based positive active materials such as LiMn2O4 or LiMnO2 have relatively good safety properties, are less costly than the other materials, and are environmentally friendly. However, these manganese-based materials have a disadvantage of a relatively low capacity. LiNiO2 has the highest discharge capacity of all the positive active materials mentioned above, but it is difficult to synthesize and it is the least thermally stable among the compounds mentioned above. LiCoO2 has many technical advantages over the other materials, such as relatively good cycle life and relatively high specific energy. Accordingly, this compound is presently the most popular material for positive electrodes of commercially available Li-ion batteries, even though its cost is relatively high.
These lithium-containing compounds are currently synthesized using a solid-phase process. For example, a lithium compound (lithium source) such as LiOH or Li2CO3 and a cobalt compound (cobalt source) are mixed at a desirable equivalent ratio followed by calcinating the mixture at a temperature of 800-1000° C. to prepare LiCoO2. A transition metal source may be added to the mixture of the lithium source and cobalt source prior to the calcination to improve charge/discharge characteristics of the LiCoO2.
Recently, with an increased demand for portable electronic equipment that is more compact and lightweight, there has been an increased demand for various types of batteries including a Li-ion battery with an improved active material that can assure good battery performance such as high discharge potential, high power density, high rate capability, and good cycle life characteristics.